What started out as a simple job to replace the decking on my deck with composite boards has now become a much bigger job, as I discovered after removing the old decking that things were not level. Suspecting something was amiss, I pounded a rod next to the 4x4 uprights, only to discover that there was nothing to stop it from going all the way down, meaning no cement had been poured around the posts. Does anybody know if there is a way to level out the support structure and stabilize it without tearing the whole thing apart and starting over, doing it the proper way? is there any way to maybe raise it up with a jack and somehow injecting concrete underneath?

You don't strictly need concreting in if they are on a firm surface. Maybe you could dig out below the post and slide a small or part cut paving slab under it. Bits of old slate or roof tile to level it off.

If its on a slope you could dig new holes adjacent and put new posts in. leave them longer than you need, then you can lever the deck up and set it at the new height by attaching it to the new posts.

You don't strictly need concreting in if they are on a firm surface. Maybe you could dig out below the post and slide a small or part cut paving slab under it. Bits of old slate or roof tile to level it off.

If its on a slope you could dig new holes adjacent and put new posts in. leave them longer than you need, then you can lever the deck up and set it at the new height by attaching it to the new posts.

Digging around it is pretty impossible as the structure is all built up around it. I'm going to try contacting a firm that seals cracks in basement walls by injecting concrete in them. I don't know if it will work, but I'll call them and see what they think.